A conventional base station is provided with an adaptive array antenna (hereinafter referred to as “AAA”) in order to suppress the effects of interference waves (such as adjacent-channel interference waves, same-channel interference waves, and delayed waves) in a received signal. A base station provided with an AAA has a plurality of antennas, and can strongly receive only an electromagnetic wave arriving from a desired direction (hereinafter referred to as “forming reception directionality”) by providing amplitude and phase adjustment for the signal received from each antenna (that is, each received signal sequence). Amplitude and phase adjustment for a received signal is carried out by multiplying each received signal sequence by a sequence-specific complex coefficient (reception weight).
A base station provided with such an AAA can strongly transmit an electromagnetic wave in a desired direction (hereinafter referred to as “having transmission directionality”) by transmitting a transmit signal multiplied by a sequence-specific complex coefficient (transmission weight) via an antenna corresponding to a sequence.
There are two main methods, as described below, for arranging antennas in this kind of base station. Firstly, there is a method (hereinafter referred to as “method 1”) whereby the antennas are arranged in positions such that the fading correlation between the signals received by the antennas is virtually 1.
When method 1 is used, a base station can obtain greater array gain both when receiving and when transmitting. For example, if the total number of deployed antennas is designated N, introducing an array antenna makes it possible to obtain a received signal that has a 2N-fold antenna gain. Also, introducing an array antenna makes it possible to obtain a received signal that has an N-fold S/N ratio.
Secondly, there is a method (hereinafter referred to as “method 2”) whereby the antennas are arranged in positions such that the fading correlation between the signals received by the antennas is virtually 0.
When method 2 is used, since the fading correlation between the signals received by the antennas is virtually 0, a base station can perform diversity reception using the signals received from the respective antennas. As a result, a base station can obtain a better received signal, with decreased fading effects, than when method 1 is used.
However, the following problems apply to a base station provided with a conventional AAA such as described above. When method 1 is used, since the fading correlation between the signals received by the antennas is virtually 1, the base station cannot perform diversity reception using the signals received from the respective antennas. Consequently, it is difficult for a base station to obtain a better received signal with decreased fading effects.
In order to prevent this kind of problem, a base station need only be provided with the number of AAAs necessary for diversity. However, in this case, the number of antennas to be installed increases, and antenna installation becomes difficult. In addition, as the number of antennas increases, the number of complex coefficients (reception weights and transmission weights) to be calculated also increases, and the scale of computation necessary for weight calculations becomes extremely large.
Furthermore, when method 2 is used, in directionality for a transmit signal, side lobes are generated in all directions rather than only in the desired direction. As a result, a mobile station located in a direction other than the above-mentioned desired direction receives major interference, and consequently has difficulty in achieving good communication. Thus, use of method 2 is not suitable for a multi-user environment such as CDMA.